Evaporative cooling towers are the most cost effective means to provide cooling for commercial air conditioning and industrial processes. From 75% to 80% of the incoming heat load to an evaporative cooling tower is removed by evaporation of cooling water. As the cooling water evaporates, removing heat from the system, the dissolved solids present in makeup water, added to replace the evaporated water and maintain cooling water level in the cooling tower, become more concentrated. At some point, the dissolved materials exceed the solubility limit(s), commonly called the saturation point, which results in precipitation and formation of undesirable scale (usually calcium carbonate).
Blowdown is water intentionally drained from the cooling tower to restrict the buildup of dissolved solids to levels below their saturation point. Cycles is a term used to denote the concentration of dissolved solids in the system water as compared to the makeup water. For instance, two cycles indicates that the dissolved solids in the system water are twice (two times) the level in the makeup water.
Blowdown constitutes a major environmental impact from cooling tower system operation as it is “wasted” water, water run to sewers that must be replaced with fresh water. For instance, a 1000 ton rated cooling tower running at two cycles will evaporate 25,000 gallons per day (gpd) with a blowdown of 25,000 gpd. If the cycles are increased to four, the blowdown would be reduced to 12,000 gpd. Basically, evaporation equals 25.0 gpd/ton cooling (one ton cooling is defined as 12,000 btu/hr) while blowdown is calculated as evaporation/cycles−1.
Cooling towers are routinely operated at two to six cycles and are generally treated with a variety of scale, corrosion, and biological control (biocide) control chemicals. As a result, cooling tower blowdown has high dissolved solids content and often contains substantial amounts of toxic materials, primarily biocides. The high dissolved solids and biocide content of cooling tower blowdown create an adverse environmental impact when discharged to the public sewers or surface waters. In addition, environmental restrictions on discharge of some active corrosion inhibitors, such as phosphate, zinc, and molybdate, have placed restrictive limits on the amount of cooling tower blowdown that can be discharged.
A small amount of water is also lost from an operating cooling tower in the air stream passing through the unit; this is commonly termed “windage” and can vary, for example, from about 0.1% to about 0.3% of the cooling water recirculation rate. Windage limits the maximum number of cycles that can be obtained in a cooling tower as it constitutes a water loss from the system. Maximum cycles are obtained at that point where windage equals the amount of blowdown. Generally, maximum cycles are limited to values between 12 and 20. Once maximum cycles are obtained, there is no blowdown from operation of the cooling tower.
Operation at higher cycles generally results in saturation limits being exceeded. As a result, acid or scale inhibitors must be added to the water to prevent scale formation. In practice, acid is not recommended due to health, safety and control issues. The use of scale inhibiting products generally limits the system to a maximum of 150 to 200 times saturation, such as taught in U.S. Pat. No. 6,645,384, herein incorporated by reference in its entirety. In most cases, the cycles obtained by use of a scale inhibitor is far less than the maximum cycles needed to obtain no blowdown and in cases of hard, alkaline makeup waters can often be as low as 2 or 3, requiring a large blowdown discharge to maintain the system scale and deposit free.
Due to drought conditions, water pollution, and continuing increased usage of fresh water supplies, many areas of the country are experiencing water shortages. In these situations where fresh water is in short supply, it is desirable to limit, or eliminate, cooling tower blowdown to conserve as much water as possible.
The United States Green Building Council (USGBC) has implemented a building certification plan for retrofitted and new buildings, Leadership in Energy and Environmental Design (LEED). The LEED certification program awards “points” for building features that improve energy usage and reduce building operation environmental impact. Reduction, or elimination, of cooling tower blowdown can provide LEED points due to reduced water use and lessened environmental impact. The USGBC LEED program is another driver towards reduction, or elimination, of cooling tower blowdown.
Many methods have been proposed for decreasing, or eliminating, blowdown from cooling towers. In one approach described in U.S. Pat. No. 4,931,187, herein incorporated by reference in its entirety, the amount of scale causing calcium added to a cooling tower is carefully controlled, by operation of a complex system of cooling water analysis, makeup softening, and controlled hard water bypass, under computer control, to maintain the cooling water saturation below a level at which scale formation would occur. This approach is costly and has proven to be impractical in practice due to analytical and control difficulties.
Another approach, as described in U.S. Pat. No. 5,730,879, herein incorporated by reference in its entirety, is to equip the cooling tower with a bypass cation resin exchanger operated in the hydrogen (strong acid) mode with bypass of cooling water through the exchanger governed by the pH of the cooling water. The rate of bypass flow is governed by the desired pH, which is selected so as to maintain the cooling water below saturation thus preventing scale as the cycles are increased. Problems with this approach involve plugging of the resin exchanger with suspended solids typically found in cooling water and the need for constant replacement of the cation resin as its acid charge is used. An additional potential problem is that if control of the bypass flow through the acid cation resin is lost, either severe scale formation will occur or acid induced corrosion of the cooling tower structure can result. A modification is given in U.S. Pat. No. 4,532,045, herein incorporated by reference in its entirety, with the addition of a bypass filter to remove suspended solids and use of weak acid mode cation resin to reduce the possibility of severe corrosion from loss of pH control. This method still suffers from the constant replacement of the cation resin as its weak acid charge is used and from control difficulties.
In yet another bypass method, U.S. Pat. No. 7,157,008, herein incorporated by reference in its entirety, describes the use of bypass chemical precipitation of hardness causing calcium from the cooling water, thus allowing higher cycles and potential elimination of blowdown. This process involves strict chemical addition of precipitating agents to the bypass cooling water flow, removal of the formed solids, and produces a liquid sludge, containing scale causing materials, for disposal. Equipment costs are quite high with this process and process control requirements are substantial.
Another method of increasing cycles to minimize blowdown is described in U.S. Pat. No. 7,122,148, herein incorporated by reference in its entirety. This process involves softening the makeup water and increasing the cycles to a point where no blowdown would be needed. No additional products are used for corrosion control. Corrosion control is due to the buildup of silica in the water by cycling and silica precipitation is prevented by maintaining a high pH in the cooling water by either natural elevation due to cycling or by the addition of sodium hydroxide. This method does not work well in current practice as cycled softened water is extremely corrosive to most materials used to construct cooling towers, in particular steel, galvanized steel, zinc, and yellow metal alloys.
None of these methods are in current common use to increase cycles to minimize, or eliminate, blowdown from evaporative cooling towers. There is a need for compositions to control and methods to prevent, corrosion and deposition within cooling towers. There is also a need for a practical process to reduce or eliminate cooling tower blowdown.